1. Field of the Invention
The present invention relates to a foamable resin composition comprising a cyanate ester, a thermoplastic resin, a blowing agent(s) and a surfactant, as well as the foamed products produced therefrom.
2. Discussion of the Background
Structural foams are known for their light weight and strength. They can be described as polymer structures which have uniform density cores and integral skins. Structural integrity requires a uniform fine cell distribution within the foam. PMI (polymethacrylimide) and PVC (polyvinylchloride) structural foams are two of the most commonly used in the aerospace industry. However, PMI foam has high affinity for moisture which will attack the foam structure and subsequently reduce the structural integrity of the PMI foam. The moisture resistance of a PVC foam is superior but its thermal stability is poor. A structural foam based on a thermoset resin and a chemical blowing agent is difficult to make due to the difficulty in expanding the resin and stabilizing the expanded cells by curing. Thermoset resins require heating to cure the resins giving the resin strength so a structural foam based on a thermoset resin must be expanded and cured at the same time. It is often that there is a mismatch between the chemical reaction and blowing temperature leading to incompletely expanded or cured foams.
Polycyanurates have reduced moisture absorbance compared to epoxy resins and polyimides. Polycyanurate thermoset compositions are finding uses in a wide variety of industrial applications, however there is a continuing effort to widen the uses for these compositions and to improve their properties, such as toughness. Polycyanurate esters have been modified with high levels of polyester carbonates (U.S. Pat. Nos. 4,157,360 and 4,334,045), acrylonitrile-butadiene copolymer (U.S. Pat. No. 4,649,714), aliphatic polyester resins (U.S. Pat. No. 4,631,319), and amorphous aromatic thermoplastic resins (EP 311341). Such modifying thermoplastic resins include polysulfones, polyether sulfones, polyimides, polyether imides, polyarylates, polyarylformals and polyaryl ethers. Thermosettable compositions based on cyanate esters have been reported of an aromatic cyanate ester, aromatic polycyanurate and an epoxy resin (U.S. Pat. No. 4,612,359) and from a compound which contains both a maleimide and cyanate group (U.S. Pat. Nos. 4,769,440 and 4,820,797).
Polycyanurate esters have been modified with thermoset resins into foaming compositions. A highly heat-resistant foam has been prepared from a polyfunctional maleimide and a polyfunctional cyanate, an epoxy resin and a carboxylic anhydride J59-226,031). Such a composition does not provide a toughened system and does not overcome the problem of moisture affinity which are characteristics of structured foams to date. It would be desirable to combine the low moisture absorbance of polycyanurate esters with the toughness of a thermoplastic polymer in a structural foam.
Despite the good mechanical properties of thermoplastic modified polycyanurate esters, no one has yet developed a structural foam exhibiting good toughness and structural properties based on a cyanate ester.